The present invention relates generally to zinc plating techniques. More specifically, it is concerned with a process for carrying out zinc plating by using a plating bath capable of forming a good plated surface in the absence of cyanic compounds.
Conventional zinc electroplating processes are generally classified into a strong alkali type process using cyanic compounds as a primary constituent and an acid type process using zinc chloride or zinc sulfate as a primary constituent. Of these, a strong alkali type plating process using a plating bath containing cyanic compounds, especially a large amount of sodium cyanide and soluble zinc compounds is most widely used at present.
It is known that such a strong alkali type plating process can provide a zinc plated surface having smoothness and semi-gloss because it uses a large amount of sodium cyanide. It is also known that if a very small amount of brighteners such as gelatin, peptone, sodium sulfide, thiourea, polyvinyl alcohol, aldehydes, ketones or salts of organic acids is added singly or together with other components to the plating bath, the resulting plated article has as even better surface.
However, because the alkali type plating process uses a large amount of cyanic compounds, which are themselves toxic materials, it is impossible to discharge the waste solution as such. For this reason, the alkali type plating process has a large number of disadvantages such as the requirement of vast facilities for treating the waste solution, the use of a large quantity of chemicals for treating the waste solution, unfavorable operation conditions, and the occurrence of public pollution. Therefore, the use of a plating bath containing cyanic compounds is nowadays impracticable from the standpoint of operative efficiency and economy.
More particularly, because zinc electroplating is applied directly onto ferrous materials in most cases, the iron is dissolved in large amounts in these zinc plating baths containing cyanic compounds. Especially, a ferro ferricyanide complex salt composed of cyan and iron is very stable and cannot easily be decomposed into free cyan. In general, a two-stage process using a chlorine method is applied for the treatment of waste water containing cyanic compounds. However, in accordance with this process, it is impossible to completely decompose the iron complex salt by using an existing treatment technique.
Under these circumstances, attention is being paid to alkali type zinc plating processes using no cyanic compound. Typical of these processes is a zinc plating process using sodium zincate and an excessive amount of sodium hydroxide. However, when zinc plating is carried out in this plating bath, a glossless and spongy zinc coating is deposited which leads to a very poor surface of the film deposited on the substrate to be plated. In order to provide excellent surfaces of zinc plated articles, attempts have been made to add brighteners to these plating baths.
It is known that examples of suitable brighteners for such alkali zincate bath are salts of glycollic acid, alkanol amines, and alkylene amines such as ethylenediamine, triethylenetetramine and triethylenepentamine. These brighteners may be used along or in combination with aromatic aldehydes. However, even if such an alkylene amine brightener is added to the above mentioned plating bath, it is difficult to form a uniform and homogeneous depoint on the object to be plated. Further, because the plating conditions for obtaining a plated surface having good gloss are strict, such a plating process is impractical for industrial purposes.
It has been also found that a reaction product of amines and epoxy compounds such as epichlorohydrin is useful as a brightener for the alkali type zinc plating bath. This brightener is primarily a reaction product of epichlorohydrin and various amines.
For example, U.S. Pat. No. 2,860,089 discloses a reaction product of epichlorohydrin and ammonia or ethylenediamine and designates the product as polyepoxyamine. Also, U.S. Pat. No. 3,227,683 describes a reaction product of epichlorohydrin and hexamine.
Although the chemical structures of these brighteners are not clearly known, it is believed that they are resins in which the epoxy ring is open and contains a hydroxyl group, secondary and tertiary amino groups, and quaternary ammonium base. These brighteners have a significantly excellent effect on plating baths containing cyanic compounds.
Attempts have been made to add such brighteners to a plating bath containing no cyanic compound in order to provide improved glossiness. However, because the resulting film has a high hardness, peeling of the deposited film often occurs during the processing of a plated article. Further, the glossiness of the film surface is not greatly improved and the deposition rate of the film is slow. For these reasons, the above mentioned plating process is feasible on a laboratory scale but is not applicable to industrial mass production. In addition, it is difficult to obtain uniformity in glossy surface as compared with a plating method using a plating bath containing cyanic compounds. Moreover, the deposit produced at a low current has no gloss. These results indicate that such a plating process is not suitable for a rack type plating operation.